Transmitting very high frequency signals between physically separated circuits can be particularly difficult, because the higher frequency signals are susceptible to a variety of different adverse influences that do not affect lower frequency signals to an appreciable degree. In general, higher frequency signals carry more information per unit of time. The amount of information per unit of time is generally referred to as the "bandwidth" or information carrying capability of the signal. If the quality of the signal is degraded to the point where the informational characteristics of the signal can not be detected or decoded accurately, the bandwidth of the signal is reduced.
It is desirable to reduce or eliminate signal degradation as much as possible, because modern computer processors, logic circuits, controllers and other information processing electrical components of electronic circuits are usually capable of operating at frequencies which are higher than those frequencies which can be reliably conducted by conventional cables and conductors which interconnect physically separate circuits. The interconnecting conductors can therefore become a limitation on the bandwidth of the system as a whole. It is for this reason that it is important to maximize the bandwidth of the of electrical conductors which interconnect physically separate high frequency electronic circuits.
Coaxial cables and microstrip flexible circuits ("flex circuits") are frequently selected to connect the physically separated electronic circuits, because these types of electrical conductors offer advantages of maintaining a relatively high bandwidth. A coaxial cable has a shielding conductor which shields each individual signal carrying conductor from exterior radiated signals and noise. The shielding prevents the external noise from interfering with the desired signal. A flex circuit establishes a controlled impedance between the signal carrying conductor and a ground or reference plane. The signal shielding and controlled impedance are each very useful in maintaining the quality and integrity of the signal, thereby achieving a greater bandwidth.
Microstrip flex circuits are usually used to connect electronic circuits which are separated by only a short distance, usually less than a meter. Coaxial cables are usually employed to carry signals over greater distances. Microstrip flex circuits are not normally connected directly to coaxial cables. If a connection is made between a microstrip flex circuit and a coaxial cable, it is usually through an electronic circuit.
Even though a coaxial cable or a flex circuit may have enhanced signal carrying capabilities, those capabilities can be significantly degraded if the connection of the coaxial cable or the flex circuit to the electronic circuit is not adequate. A faulty connection to the circuit board can reduce or compromise the bandwidth of the high frequency signals just as much or more than a limited bandwidth resulting from the conductor itself.
The typical technique of connecting a coaxial cable to an electronic circuit is with a terminating connector. The terminating connector includes a center conductor to electrically connect a center conductor of the coaxial cable to the conductor traces of the circuit board. An exterior mechanical connector device, such as a threaded nut or a mechanical friction fit retaining device, electrically connects the shielding conductor to the ground reference of the electronic circuit and mechanically holds the coaxial cable in place. The threaded nut or friction fit retaining device occupies a relatively large amount of physical space at the edge of the electronic circuit, thereby limiting the number of connections which may be made in a given space.
In those situations where a relatively large number of coaxial cables must be connected to a circuit board of a relatively small size, the physical space requirements for mechanically connecting the coaxial cables may be greater than can be accommodated. In those cases, the shielding conductors of two or more coaxial cables are typically connected to a single mechanical device, thereby gaining some additional space. One typical approach to consolidate the shielding conductors has been to connect the shielding conductors of two adjacent coaxial cables and then insert the two connected shielding conductors in a ground plane receptacle. Another approach has been to solder a jumper wire to the connected shielding conductors and then insert single jumper into the ground plane receptacle. As a result, only three connections are required to interconnect two coaxial cables to an electronic circuit. The number of conductors or the "signal density" of connections is thereby raised.
The disadvantage of connecting the shielded conductors of two or more coaxial cables to the ground plane at a single solder connection or with a jumper is that the bandwidth of the coaxial cable is usually reduced as a result of this connection. The connection of the shielding conductors at the location where they are joined together creates relatively high inductances, resulting in signal path discontinuities, which reduces the bandwidth of the coaxial cable.
One of the disadvantages of connecting a multi-layer microstrip flex circuit to an electronic circuit board involves the connection of the inner ground plane conductor and the interior microstrip traces to the electronic circuit. The typical approach is to form individual connection pads on the exterior of the flex circuit and connect the connection pads to the interior ground plane and to the interior microstrip traces with plated through holes known as "vias". The vias route signals from the conductors inside the flex circuit to the outside connection pads. The vias and connection pads are then soldered to bonding pads of the electronic circuit.
To keep the impedance of the via to the connection pad controlled, each via must be very small in size, such as on the order of 0.005 inch. Making vias this small is very difficult. Consequently larger vias are typically employed, even though the larger vias typically introduce signal discontinuities and reduce the bandwidth of the signals conducted by the microstrip flex circuit.
It is with respect to these considerations and other background information that significant improvements in the field of interconnecting coaxial cables, microstrip flex circuits and electronic circuits have evolved.